1997 S-10 Blazer 4dr 2wd LS . It recently had a new heater core put in,
heat worked great for a week or two then it went back to blowing out
cold air. So i replaced the t-stat and radiator cap then i flushed the lines
an im still getting cold air.

 

poss air pocket in the heater hoses.. i had that same prob on my 1993 chevy van

 

Is the rad full?
Are both heater hoses the same temperature or is one hot and the other cold?

 

Do you get heat out of any other modes?

 

I checked the radiator it was full and circulating, not getting heat in any other mode either, heater hoses are both the same temp ,neither seem to be very warm.

 

A thermostat is fairly inexpensive and may well be the cure then.
If it's stuck open, then your engine wont reach the optimum temperature.

 

A NEW T-STAT WAS PUT IN THE SAME TIME THE HEATER CORE WAS CHANGED.

OH YA I ALMOST FORGOT . WHILE IN THE PASSENGER SEAT YOU CAN HEAR FLUID DRAINING ANY TIME YOU CHANGE MODE FROM DEFROST TO FLOOR OR ANY OTHER MODE . I KNOW ITS NOT LEAKING ,ITS LIKE YOU CAN HEAR IT DUMPING INTO THE HEATER CORE.ITS REALLY WEIRD. [align=right][/align][align=right][/align][align=right]
[/align]

 

OH YA I ALMOST FORGOT . WHILE IN THE PASSENGER SEAT YOU CAN HEAR FLUID DRAINING ANY TIME YOU CHANGE MODE FROM DEFROST TO FLOOR OR ANY OTHER MODE . I KNOW ITS NOT LEAKING ,ITS LIKE YOU CAN HEAR IT DUMPING INTO THE HEATER CORE.ITS REALLY WEIRD.

 

You have my attention, the caps are not nessessary.

The sound you hear in the heater core is probably air. If you have air in the system, you will not have sufficient heat.
I know that you said you had a new thermostat but I have installed faulty thermostats in the past on my own vehicles.

 

My blazer had the same problem, not blowing any heat. After flushing out the heater core, new thermostat and many people telling me it's the temp control sensor in the dash, found out my blend door actuator was stuck. One quick fix later and now all I have is heat(not a problem for now) but I need to replace it, online parts around $35. Dunno if this will help but it's another option to look at.